Post on 15-Aug-2020
transcript
Drawing Skeletal (Zig-Zag) Structures
The quick and easy way to draw organic molecules
Information Overload vs Quick and Easy
• In a line-bond structure you see EVERYTHING (except for lone pairs, actually).
• All atoms must be drawn into the structure.
• Ex:
• These can take a long time to draw!
C C C C C
C
H
H HH
H
H
H
H
H
H
H
H
H
H
Which is cleaner and more concise?
• Skeletal structures are perhaps a little confusing… Seems like things are missing…
• Once you know the rules, skeletal structures are actually much easier to draw!
OR
skeletal
CC
CC
CC
C
CC
H
H H
H H
H H
H
HH
H
HH
H
H
HH
HH H
linebond
Skeletal Structures
• Skeletal structures are those “zig-zag” structures you see quite often.
• “Zig-zag” is required so you can see connectivity… lines that are “straight on” may be confusing:
vs
(there are 2 here!)
Skeletal Structures - Rules
• In order to understand HOW to draw molecules using these zig-zag lines, you need to follow a certain set of rules, or else none of it makes any sense
• We will start by converting to line-bond structures that show everything.
Skeletal Structures – Rule #1
• Rule #1: never draw a “C” to represent a carbon atom (as in C-H or C-C or C=C…)
• When doing shorthand notation like this, “less” is faster to draw, so ditch those “C”s!
Skeletal Structures – Rule #2
• Rule #2: At the end of any line, you will always assume there is a C, if no other atom is shown.
• Take this single line, the simplest skeletal structure possible:
• How many carbons do you “see”?
Skeletal Structures – Rule #2
• If the end of a line represents a carbon atom, then you will “see” a carbon at each end of the line:
• That line represents:
C C
Skeletal Structures – Rule #3
• Rule #3: At the intersection of two or more lines, assume there is a C, if no other atom is shown.
• Now take this skeletal structure:
• How many intersections are there?
Skeletal Structures – Rule #3
• There are two lines connecting in the center to form one intersection:
• That intersection represents a carbon atom, without having to draw the C’s.
• Up to four lines may connect to intersect.
Skeletal Structures – Rules 2 and 3
• How many total carbons are in this molecule?
• You have to count all intersections and the ends of any lines to get the total number of carbons represented.
Skeletal Structures – Rules 2 and 3
• So, how many total carbons are in this molecule?
• One intersection plus two ends of lines adds up to three total carbon atoms:
C
CC
endend
intersection
Skeletal Structures – Rules 2 and 3
• How many total carbons are in this molecule?
Skeletal Structures – Rules 2 and 3
• How many total carbons are in this molecule?
• Five carbons total:
end
end
end
intersections
CC
CC
C
Skeletal Structures – Rules 2 and 3
• One more time, how many total carbons are in this molecule?
Skeletal Structures – Rules 2 and 3
• One more time, how many total carbons are in this molecule?
• Five end carbons…
end
end
end
end
end
Skeletal Structures – Rules 2 and 3
• …and four intersecting carbons…
• …for a grand total of 9 C’s you didn’t have to draw!
CC
CC
CC
C
CC
Skeletal Structures – Rule #4
• Rule #4: The “H” of a hydrogen attached to carbon is not drawn.
• Just remember that carbons must have four bonds. Count bonds and subtract from 4 – that will be the number of H’s.
• Take this skeletal structure again:
• How many hydrogen atoms are on each carbon?
Skeletal Structures – Rule #4
• Recall that there are C’s at the end of each line.
• The left-hand C has one bond (to the right-hand C). This means that, by default, it must have 3 hydrogen atoms attached (4 total minus 1 to a C = 3 H)
• The right-hand C also has one bond to a C. This means that it too also must have 3 hydrogen atoms attached (4 total minus 1 to a C = 3 H)
C Cequals
Skeletal Structures – Rule #4
• Final structure?
• The skeletal structure on the left was WAY easier to draw… With practice, you’ll get used to this process…
C Cequals equals C C
H
H
H
H
H
H
Skeletal Structures – Rule #4 again
• Take this skeletal structure:
• How many hydrogen atoms are on each carbon?
Skeletal Structures – Rule #4
• Left carbon – one line
• Right carbon – one line
• Center carbon – two lines
• Left carbon – 4-1 = 3 H
• Right carbon – 4-1 = 3 H
• Center carbon – 4-2 = 2 H
Skeletal Structures – Rule #4
• Equivalent structures
equals
CC
C
H H
H
HHHH
H
Try another molecule
• Convert the following skeletal structure to a line-bond structure:
• Add C’s to “ends” and “intersections” and then determine how many H’s are attached to each. Don’t move forward until you’ve drawn it!
Answer?
• These two are the same molecule:
equals CC
CC
C
C
C
H
H
H
H
H
HH
H
H H
HH
H
H
HH
Answer?
• Remember that your answer may look similar but not exactly the same.
• What counts is that you have the C’s labeled correctly and you have the right number of H’s on each C. For instance, my C(#1) has to have 3H’s, C(#2) has to have 2 H’s, C(#3) has to have 1 H, etc…
equals CC
CC
C
C
C
H
H
H
H
H
HH
H
H H
HH
H
H
HH
1
23
6
4 5
7
12
Skeletal Structures - Rule #5
• Rule #5: Everything besides C-H and C-C must be shown. These other atoms (like O, N, F, Cl, Br, etc) must be shown.
• Note that Hydrogen atoms can and should be shown for these other atoms and even C=C has to be drawn, even when C-C does not.
OH O
Line-Bond to Skeletal Structure
• Now that you have a sense of what skeletal structures equate to, let’s try the other direction…
• A skeletal structure is a line-bond structure without its letters.
CC
CC
C
C
C
H
H
H
H
H
HH
H
HH
HH
H
H
HH
Line-Bond to Skeletal Structure
• So you need to simplify. Start by removing all those H’s on the C’s…
C
CC
CC
C
C
H
H
H
H
H
HH
H
HH
HH
H
H
HH
CC
CC
C
C
C
Line-Bond to Skeletal Structure
• Then erase all those C’s…
• Good job… Try the next one!
CC
CC
C
C
C
Line-Bond to Skeletal Structure
• Convert the following to a skeletal structure:
• Erase the C-H bonds, then the C’s…
C
CC
C
CC C
CBr
H
H
H
H
H
H H
H H
Line-Bond to Skeletal Structure
• Leave the Br though!
C
CC
C
CC C
CBr
H
H
H
H
H
H H
H H
C
CC
C
CC C
CBr
Br
Line-Bond to Skeletal Structure
• Convert the following to a skeletal structure:
CC
CC
CC
Cl
C
H
HH
H
H
HH
H
H
HH
HH
Line-Bond to Skeletal Structure
• Erase the C-H bonds, then the C’s… but leave the Cl!
Cl
CC
CC
CC
Cl
C
H
HH
H
H
HH
H
H
HH
HH
CC
CC
CC
Cl
C
And in the other direction…
• Obviously, you need to put the letters back into place, alone with the C-H bonds…
• Draw the Line-Bond structure for:
• Find ends and intersections first…
Skeletal to Line-Bond…
• Ends in blue… intersections in red…
• Triple bonds are a bit confusing at first – the intersection is actually straight, when drawn correctly:
So, put in the C’s…
Skeletal to Line-Bond…
• And now you have:
• Now add in the C-H bonds. Every C must have a total of four lines.
CC
CC
CC
Skeletal to Line-Bond…
• Finished Line-Bond Structure:
• Notice how the one end of the triple bond, the red carbon, already has four bonds so no bonds to H for that carbon!
CC
CC
CC
HH
H
H
H H
H H
H H
Skeletal to Line-Bond or V.V
• These take practice… Once you’ve mastered the basics of the skeletal structure you are ready to make the leap to converting skeletal structures to condensed formulas and back again…
• When you are ready, go check out the next PowerPoint – Skeletal to Condensed and Back Again